The invention relates to a measuring roll for continuous measuring of the distribution of the stresses applied to a band, useable especially for detecting the flatness defects of a sheet metal.
When rolling metal bands and notably cold-rolled thin sheets, flatness defects appear, caused in particular by small variations over the width of the band, in the elongation produced by rolling.
There result flatness defects that can be corrected while acting on the rolling conditions and, in particular, in the transversal direction, of the clamping pressure applied between the working rolls.
In modern mills, indeed, it is possible to act locally on the distribution of the loads, for example by a thermal effect, by means of zones spraying systems, or by mechanical effect, while exerting curving loads on the ends of the working rolls. It is also possible to use a back-up roll comprising a deformable envelope mounted to rotate round a fixed shaft and resting on the said shaft by a number of actuators that may be adjusted in position and in pressure.
In order to detect the defects to be corrected, the band is made to run over a measuring device, placed downstream the mill, and sensitive to variations, over the width of the band, of the tension load applied to the said band.
Such a measuring device is composed, usually, of a deflector roll comprising a cylindrical body mounted to rotate round an axis perpendicular to the longitudinal running direction of the band that is applied under stress over an angular sector of that roll.
A number of sensors distributed on the external cylindrical face of the roll enable to measure the variations of the local application pressure of the band. Normally, these sensors are spaced apart from one another regularly and distributed over the whole length of the roll, whereas the band can be divided into a series of adjacent zones of width corresponding to the transversal spacing between the sensors.
On the other hand, to avoid any interference between the measures conducted on two adjacent zones, the sensors are advantageously staggered by a certain angle from one zone to the next.
In a known embodiment described, for example, in the document AT-B-370521, the deflector roll is composed of juxtaposed annular segments each composed of at least one load sensor located in a housing provided in the thickness of the segment and made of a piezoelectric transducer interposed between the bottom of the housing and a protection part that closes the housing outwardly.
Such an arrangement enables good dissociation of the measuring zones of the band, but it is difficult to ensure perfect junction between two adjacent segments. Therefore, there are risks of marking the band due to impurities build-ups along each jointing plane between two segments.
According to another embodiment, described in the document JP-A-62-047529, the deflector roll is composed of a tubular body in which are drilled a number of radial bores each forming a housing for a cylindrical casing whose bottom consists of a thin wall that is tangentially aligned with the external face of the tubular body. A magnetostriction-type sensor is applied to the internal face of that wall.
Thus, in both cases, each measuring zone is associated with a load sensor on which pressure applied by the band is transmitted by the closing wall. The inherent resistance of that wall, which is interposed between the sensor and the band may affect the measure of the pression in a manner which is difficult to appreciate. Besides, the measure can be disturbed too by external influences resulting for example from the expansion of the roll due to the temperature of the band or its deformation under the effect of the loads applied which may induce jamming of the protection wall.
To avoid these perturbations, a small clearance can be left at the periphery of the closing wall, but there is still a risk of marking caused by the build-up of impurities along the edge of the wall.
Such drawbacks may be avoided with another type of flatness measuring roll which has been developed by the applicant since many years, such a roll using the deformation of the protecting wall for determining by calculation the variation of the pressure applied by the band.
Thus, instead of disturbing the measure of the pressure, the wall covering the sensor becomes a means for directly measuring these pressure.
To this aim, we do not use load sensors but displacement sensors measuring directly, in each sensitive zone, the deflection resulting from the pressure applied. Then, by applying the formulas of Resistance of Materials, it is possible to determine by calculation the variations of that pressure from the measurement of the deflection and this calculation does not risk to be affected by external perturbations, since the measuring feelers can be calibrated while performing a blank measurement in order to take into account solely the deformation caused by the loads applied by the band.
On the other hand, to avoid any risks of marking, it has been suggested to cover the whole roll with a continuous surface. In such an arrangement described, for instance, in the European patents EP-028.191 or EP-0.270.442 of the same company, the deflector roll comprises a central body composed of a tubular wall that is thick enough to confer the necessary resistance and on which is attached a continuous thin envelope that is, generally applied by hooping, whereas the sensors are located in bores provided in the central body, and closed outwardly by the corresponding section of the thin envelope.
This section of the envelope forms a sensitive zone and it can be considered generally that the said zone behaves like a thin plate, normally circular, and fixed along its periphery, whereas this plate is subject to uniform pressure. We now that, in such a case, the deflection in the centre of the plate is given by a formula of the type:
f=KpD2/e3xe2x80x83xe2x80x83(1)
where D is the diameter of the plate and (e) its thickness.
As the measuring zone is sized for a determined sensitiveness, it appears that the deflection (f) measured in the centre of the plate by the feeler is proportional to the pressure applied (p) that is itself proportional to the traction of the band, in the corresponding zone.
On each measuring zone, the deflection is measured by comparing in the positions of the mobile element of the feeler in the centre of the plate, respectively when empty and when loaded. It is thus possible, while comparing the measurements conducted by the different sensors, to determine the variations of the traction load in adjacent zones distributed over the width of the band.
Using a thin envelope covering the whole roll advantageously eliminates, as seen, any risks of marking. However, mounting the said by hot hooping is rather complex.
On the other hand, deterioration and wear of the external face of the roll, on which the band is applied under traction and runs at high speed, cannot be avoided. Periodically, it is thus necessary to rectify this face and even to replace the envelope in case of excessive wear and, to do so, first of all, all the sensors, must be disassembled. Even if we use an assembly on removable bars that is subject to the European patent 0.270.442 already mentioned, such operations are complex and costly since it is generally necessary to remove the deflector roll for a rather long period, which causes a production stoppage.
The purpose of the invention is to remedy these shortcomings thanks to a new arrangement enabling simplification of the design of the roller and of the maintenance operations and providing, in particular, the opportunity of intervening punctually on either sensor without dismantling the assembly.
The invention relates therefore generally to a roll for measuring the distribution of the loads on a band running along a longitudinal direction and applied to the said roll, the said roll comprising a tubular body mounted to rotate round an axis perpendicular to the running direction and carrying a number of measuring members sensitive to a local application pressure of the band and each mounted in a housing limited by a lateral face, closed outwardly by a wall with an external cylindrical face of same curving radius as the external face of the roller, whereas each measuring member is a feeler-type displacement sensor comprising a mobile element mounted to slide radially over a fixed element and applied outwardly to the internal face of the closing wall of the housing.
According to the invention, each housing of a sensor is closed individually by a hollow part in the shape of a lid with a bottom made of an elastically deformable thin wall and integral with a peripheral part in the form of a bushing delineated by two lateral faces, respectively internal and external, whereas the external face exhibits a profile, in cross section, identical with that of the lateral face of the housing, so that the lid is encased without any clearance into the said housing, in a position for which the external face of the bottom is situated in the extension of the external face of the roller, the lateral opposite faces of the housing and of the bushing being applied one onto the other along a sufficient height so that the said bushing forms a peripheral embedding means capable of resisting the deflection moments resulting from the pressure applied on the thin plate by the band, without any risks of separation along the joint between the external face of the lid and the external face of the tubular body.
In a particularly advantageous fashion, the internal lateral face of the embedding bushing connects, by a rounded fillet, to the internal face of the deformable plate on which the mobile element of the feeler rests.
According to a preferred embodiment, each lid of a housing is associated with a means for permanent application of lateral faces of the housing and of the bushing which are opposite to each other.
In a particularly advantageous fashion, this maintaining means is composed of a thrust part placed in the housing and protruding inside the bushing of the lid, whereas the said thrust part is delineated by a lateral face with an identical profile, in cross section, with that of the internal lateral face of the bushing, in order to provide between the said thrust part and the lateral face of the housing, an annular space in which the bushing of the lid is encased without any clearance, inwardly and outwardly.
According to another preferred characteristic, the thrust part is delineated, outwardly, by a front end face, spaced from the internal face of the ductile thin wall by a slight clearance corresponding to a maximum deformation in the elastic range of the thin wall under the effect of a local application pressure of the band, and on which rests the said thin wall in case of excessive pressure, whereby the said front face is drilled with a passage orifice with a certain play of the mobile element of the feeler.
On the other hand, the thrust part can be fitted, at the bottom of the housing, with a flared section forming a peripheral rim on which rests the embedding bushing of the lid, in the engaged position of the said lid for which the external face of the thin wall lies in the extension of the external face of the tubular body, whereby the internal lateral face of the bushing is delineated by a groove exhibiting an annular bottom on which rests the peripheral rim of the thrust part. Thus, the embedding bushing of the lid bears against the bottom of the housing at its internal lateral face, which promotes the embedding effect.
Thanks to these arrangements, the lid can be embedded simply with a little strength in the annular space between the thrust part and the lateral face of the housing. However, the lateral faces opposite the embedding bushing of the lid and of the thrust part can be glued to one another, whereas both parts can be easily disassembled as a whole in order to allow for punctual intervention in a measurement area without having to disassemble all the feelers.
Other advantageous characteristics are the subject of secondary claims.